1. Field of the Invention
The present invention relates to the measurement of physical parameters, such as strain, pressure, acceleration, flow rate and force by fiber optic sensor utilizing the photoelastic effect. More particularly, the invention pertains to utilize in the optic sensor specific strains of single-mode and graded-index multimode optical fiber, such as radial compression and twisting, which effectively produce the photoelastic effect. The present invention also relates to associated data acquisition systems.
2. Description of the Related Art
Fiber optic sensors successfully substitute now conventional resistance strain gages that have been the most widely used in the past, and are the most readily available technology at this time. The sensitive element of fiber optic sensors is a length of specially prepared optic fiber that alter the transmitted optical signal in a manner that can be detected and measured by optical instrumentation under a stress applied on the fiber.
This optical sensor technology has overcome many of the inherent disadvantages of resistance strain gages and its electrical transmission networks, including long-term measurement drift, sensitivity to electromagnetic interference, and dangers from electrical power requirements, which have limited their application in certain fields, such as fire and explosion hazardous environments.
Within the past two decades, a number of manufacturers have attempted to exploit this fiber optic sensor technology, with limited results. The costs and complexity associated with the electronic and optical systems required to implement the fiber optic sensors were prohibitively high for most applications. Successful development of fiber optic technology for telecommunications industry has greatly mitigated this problem.
There are three main principles that are now utilized in strain fiber-optical sensors:                Fiber Bragg grating. It is the most developed technology used for fiber-optical stress sensors. Here, the single-mode optical fiber with written Bragg grating is stretched by external force. It causes deviation of grating period that can be measured. For example, if a tensile force of 1 kg is applied to standard single-mode 9/125 optical fiber, it will be stretched on 0.82% that shifts the light wavelength reflected from the grating on about 39 nm at 1550 nm. There are varieties of measuring technology that allow measuring such wavelength deviation. In some designs, such fiber sensors have doubled or tripled grating and utilize multi-wavelength light source to achieve more reliable measurements.        Fabry-Perot interferometer. Here, strain (tensile force) applied to the fiber stretches it. Usually, such sensors have an optic fiber with a small gap, and under a tension applied to the fiber the width of the gap are changed. So, the wavelength of the light reflected from this gap is shifted that can be precisely measured.        Scattered light sensors (Brillouin scattering). Another physical principle utilized in the strain sensors is the light scattering. Here, the strain measurement is based on a variation of scattered light produced by an incident light launched into the fiber. The strain applied to the silica fiber changes structure of the material so changes the back reflection caused by the scattering.        
The review of the existed fiber optic strain sensors, its principles and designs reveals that the sensors mentioned above have some disadvantages, such as high cost and complexity of measuring equipment. In some cases, the data acquisition systems utilizing these sensors have problems with the measurement instability, temperature and polarization sensitivity (especially, interferometer-based sensors). Also, measuring system implementing fiber optic strain sensors based on Bragg grating technology requires complicated multi-line single mode light source that has to provide large number of spectral lines, or wavelength scanning light source.
Therefore, for the mass applications, such as monitoring of structural elements, pipeline pressure and flow rate, it is necessary to develop a low cost, simple design, stable in different ambient conditions and reliable fiber-optical strain sensor based on alternative approach. Also, the data acquisition system based on these sensors has to support a number of sensors and provide data compression possibility that allows using a single optical line to monitor a number of sensors.
There is another principle that a fiber-optical strain sensor could be based on. It is the photoelastic effect, phenomenon of polarization conversion of the light running in optic fiber under stress applied to the fiber. It usually appears in conventional single-mode optical fiber and creates serious problems for the newest telecommunication lines using polarization-sensitive equipment.
Generally, a conventional single-mode optical fiber can not maintain polarization of light signal running in the fiber. Some external mechanical forces, even small ones, applied to the fiber can induce a birefringence in the fiber core so converting input polarization. For example, if a radial pressure is applied on the portion of the fiber, it produces a linear birefringence there with its slow axis in the direction of applied pressure. The retardation between the slow and fast axes can be varied from 0 to 2π by changing of applied pressure. Therefore, a strain appearing in the fiber core under external mechanical forces applied on the fiber produces a kind of variable size retardation plate (depends on applied force) that converts light polarization. Such retardation plate induced in the fiber core can work as ½, ¼-wavelength plate or its combination rotating polarization plane of input light, transforming linearly polarized light into elliptical one and vise-versa. This deviation of the light polarization can be measured using a polarization analyzer. The intensity of the light sequentially passed through the polarizer & analyzer system can be calculated according to the formula:I=Io Cos2 φ,
Where Io—is the amplitude of input light,                φ—is the angle between polarizer and analyzer polarization planes.        
Therefore, if angles of the analyzer and input light polarization planes are aligned, and there is no stress applied on the fiber, the light passes the analyzer without attenuation (I=Io). When the stress is applied, the angle of polarization plane of the light running in the fiber turns, so intensity of the light passed the analyzer declines (I<Io). Thus, variation of the stress produces amplitude modulation of the output signal that can be utilized for the strain measurement.
This effect is used in a fiber Babinet-Soleil polarization compensator developed for fiber-optical telecommunication lines. It contains a piece of bare single-mode optical fiber and squeezer, the mechanism that can be tightened and rotated about the fiber so converting any input polarization into desired one. The device, despite its simplicity, is very reliable and keeps this conversion for a long time without deviation.
General idea of utilization of this effect for strain measurements was claimed, particularly, in U.S. Pat. No. 4,173,412 granted to M. Ramsay and S. H. Wright, and U.S. Pat. No. 4,564,289 issued Jan. 14, 1986 to Spillman, however, it should be noted that the optic sensor suggested by the patent author does not specify the stress that has to be applied to a single mode optic fiber to produce high birefringent effect. In the preferred embodiment of U.S. Pat. No. 4,564,289, the author utilizes an axial stretching of the fiber—the strain that is suitable for the Bragg grating sensors, but does not produce high birefringent effect. Moreover, in the preferred embodiment shown on FIG. 1 of U.S. Pat. No. 4,564,289, the circular polarized light, which is proposed by the author to feed the sensor, and elliptical polarized light carrying the information about the strain (line 105 and 106 on FIG. 1 of the patent) can not be properly transmitted neither by conventional single mode fiber-optical line, nor by polarization-maintaining fiber line without polarization distortion. Also, light emitting diode proposed in U.S. Pat. No. 4,564,289 can not be properly matched with single mode fiber, and multi-wavelength light, also proposed there, causes different polarization variation for each wavelength so making measurement less reliable. And, the idea to use an analogue differential signal developed by special sensing unit, such as differential bridge, looks obsolete, because it comes from old style electric strain gages and analogue differential amplifiers. Modern computer technology permits processing sensing and reference elements separately that allows drastically simplifying the sensor design and associated measuring system.
Theoretically, there are four possible stresses that could be applied on an optical fiber, such as axial stretching, bending, radial compressing and twisting, but only two of them—radial compressing and twisting—produce high birefringent effect.
The idea of utilization of these effects to measure a strain was proposed in U.S. Pat. No. 6,211,962 B1 issued Apr. 3, 2001 to Nolan. The author of this patent suggests a polarization-maintaining single-mode optic fiber as a sensing one. He connects lengths of such fiber in sequences, wherein each length is affected by different phenomenon, such as pressure, temperature, etc. To separate readings of each parameter, the author of mentioned above patent suggests launching a multi-wavelengths light, wherein each wavelength represents single measured parameter. To achieve amplitude modulation of the light passing the fiber, the author suggests launching two single-mode lights having orthogonal polarizations.
The experiments conducted by the author of the present invention reveal that a polarization-maintaining fiber is not the best sensing element of such sensors. Any deviation of polarization angle of a single-mode linear-polarized light running in polarization-maintaining fibers transforms the linear-polarized light into elliptical one, which amplitude is not in proportion with measured phenomenon.
According to optics, two orthogonally polarized lights can not interfere; the interference occurs in linear-polarized light only when planes of the polarization are aligned. Interaction between two orthogonally polarized lights only rotates vector E of electromagnetic wave—the effect known as elliptical or circular polarization. To transform elliptically-polarized light back into linear one the polarization compensator—quarter-wavelength plate—is used. Also, amplitude of the combined light does not change when a linear-polarized light is transformed into elliptical one. In reality, many parts of conventional single-mode fiber-optical telecommunication line is affected by some mechanical deformations, such as bending, twisting, compressing, etc., which randomly induce birefringence along the line. As the result, in long single-mode optical lines this effect many times transforms initially linear-polarized light into elliptical and back into linear one also changing its polarization angle, but these variation of polarization state does not introduce any attenuation of the output signal; only ratio between two orthogonally-polarized components is changed. Thus, to achieve any reading, it is necessary to sequentially install the quarter-wavelength retardation plate converting elliptically-polarized light back into linear one and, also, a polarization analyzer that transforms deviation of polarization angle into light amplitude modulation.
Introducing the second light with orthogonal polarization angle proposed in U.S. Pat. No. 6,211,962 B1 makes the situation more complicated, because these two running lights initially produce elliptical or circular polarization; and each component reacts differently to applied stress. The multi-wavelength light proposed in U.S. Pat. No. 6,211,962 B1 can produce complete mess, because each length of affected sensing fiber introduces birefringence that sequentially changes polarization state of the light of each wavelength passed all stages. It was mentioned in U.S. Pat. No. 6,211,962 B1, but separation of these effects proposed by the author of this patent is very problematic, even though it was mathematically treated. Also, in the case of multi-wavelength light, polarization converters—the quarter-wavelength plates—can not provide accurate conversion because of mismatching of wavelengths. Theoretically, such system can be initially aligned (zero reading), but, when measured phenomena start affecting, the system becomes completely misaligned.
The feasibility of polarization-maintaining fibers as the sensing ones was investigated by the author of the present invention. The research reveals very unreliable reading of the sensors utilizing this kind of optic fibers. Because of this, the author of the present invention refuse utilizing polarization-maintaining optic fibers as sensing ones, and use it to delivery linear-polarized light to sensing fibers only.
Unlike the polarization-maintaining fiber, conventional single-mode optic fiber provides the most reliable and stable conversion of applied stress into rotation of polarization angle of single-mode linear-polarized monochromic light that can be reliably measured. Reliability and stability of conversion has been proved by experience with fiber Babinet-Soleil polarization compensator utilizing radial compressing and twisting stresses applied on a length of conventional single-mode optic fiber.
Another kind of optic fibers that can be used in polarimetric strain sensors is graded-index multimode optic fibers. The research conducted by the author of the present invention reveals that these fibers, unlike step-index ones, can transmit single mode light with low losses and without conversion it into multi-mode one. Moreover, radial compressive or twisting stresses applied to a graded-index multimode optic fiber produce high birefringent effect similar to one induced in single-mode optic fibers.
The fiber optic sensors of the present invention utilize radial compressing and twisting stresses applied to length of a single-mode optic fiber; and when the sensor is affected by measured parameter, such as strain, pressure, acceleration, gas or liquid flow, the optic fiber realizes corresponding compressing or twisting stress so producing measurable polarization angle rotation that is in proportion with the measured parameter.
Another object of this invention, a data acquisition system collecting measurements performed by the fiber optic sensors, is based on novel schematic solutions and fiber optic polarization equipment developed for newest polarization-sensitive fiber-optical telecommunication lines. It includes all-optical time-division multiplexing units, such as MEMS photonic switch or Acousto-Optical Switch for Fiber-Optic Lines described in U.S. Pat. No. 6,539,132 issued Mar. 25, 2003 to G. Ivtsenkov at al.